What Happens:
B-air's moderate knockback would place your opponent a fair distance away, depending on %.
Unless you SH'ed this B-air, there would be no time to reach the ground, even fast falled, before the opponent had a chance to get up. You also need to complete a turnaround, or reverse fireball before you are facing the right direction. Although this only takes an extra couple of frames, you are just giving your opponent more and more time.
Also the more time you are giving your opponent the more time they have to either
1:Get up (have a chance to knock away the fireball with the get-up attack)
2: Roll out: Which could take them out of range of the fireball
But yeah, let's assume you SH'ed the B-air...
The get knocked back about, 3/4 the length of a fireball
(lets assume, as percentages are unknown).
You then either:
1. Land, turnaround and shoot some green
2. Attempt a reverse fireball out of the SH'ed aerial
In either of these situations the only problem is the fireball. It is a fairly slow moving projectile and unfortunately alot of characters have get-up attacks that will cancel if out, or rolls that will move them out of range.
Furthermore, if the fireball hits, there is still problems.
Assuming
(sorry, I am assuming again) that they were originally knocked back that 3/4 range I really doubt that Luigi can get there in time before they perform a roll or get-up attack.
There is considerable lag after the fireball is shot, and Luigi's running speed....well it's just not Sonic is it?
I personally haven't tried this, but I am about to jump into training mode and test it on a dummy. In theory, it is a workable move, but it requires a very specific response from your opponent. In any other response, the fireball will get blocked, rolled away from or you will miss your FJP and get punished.
That's just my view of this scenario, please don;t take it for granted...lower percentages might prove different, but then again they may not. I will edit once I know...
EDIT:
Result of Training Mode:
So I checked it out in training mode, and if you can actually get a person to hit the ground before having a chance to air-dodge or attack out, then kudos to ya. The angle in which the B-air projects the opponent
(tested on Marth btw) makes it fairly impossible for them to land on their faces. If they can, I haven't seen it.
So then I move on to D-air spiking, which some people mentioned before. I tried it at past 50% so they would actually get knocked down far enough and:
At most percentages, until you get to ridiculously high ones, they still have enough time to get up as you land, and way before you have a chance to shoot a fireball.
Realisticly, the best option you should have if you SH'ed the D-air is a buffered d-smash or f-smash I guess, depending on whether they rolled to your opposite side.
Btw, just because I couldn't get this to work doesn't mean it's impossible. By all means, test it out for yourself, play around and let me know..
Further Training Mode Stuff:
Okay, so I've got the opponent knocked over with a b-air and I've fireballed him and I've FJP'ed him. So now the theory works. Only problem is, the computer lies on the ground alot longer than a real opponent would. Problem two, the opponent was Fox, so he is a fast faller, so it was easy to get him in the state.
So in theory this does work, it is possible to ground them with a B-air, but there is still time to roll away or get up attack.
It may be useful as a surprise once, but once again, the opponents response has to be to lay down for that half a second longer.
You need to hit them with a certain part of the bair, at a certain spot for it to work.
A normal Sh'ed B-air is fine, but the percentage at which it is done is absolutely crucial. It would also be different for every character, so although this can work in theory it is very situational. I definitely felt good when I pulled it off though. I'll try using it in some matches and we'll see if the opponent reacts how I want them to.